CCD image sensors capture images through photosensitive sites that generate charge in response to the intensity of light striking the photosensitive sites. FIG. 1 is a top view of a CCD image sensor in accordance with the prior art. Pixel array 100 includes rows and columns of pixels 102. Each pixel includes a photosensitive site (not shown) that generates charge in response to incident light striking the photosensitive site. To read the accumulated charges out of pixel array 100, a vertical CCD shift register (not shown) is either positioned adjacent to, or contained within, each column of photosensitive sites to receive and shift the accumulated charges to horizontal CCD shift register 104. The charges are shifted through the vertical CCD shift register in a row-by-row or in parallel process. Horizontal CCD shift register 104 receives the charges from the vertical CCD shift registers and sequentially shifts each row of accumulated charges to output amplifier 106 in a pixel-by-pixel or serial manner.
The CCD shift registers are emptied of all non-image related charges, such as dark current, prior to image exposure and readout of the accumulated image-related charges. This is known as a reset or flush operation. Resetting the CCD shift registers prior to image capture allows an image sensor to output an accurate representation of a captured image. FIGS. 2(a)-2(b) are graphical illustrations of a reset operation in a CCD shift register in accordance with the prior art. Non-image or unwanted charges 200 collect in CCD charge storage regions 202, 204, 206, 208, 210, as shown in FIG. 2(a). A reset operation is performed to cause non-image charges 200 to transfer from charge storage regions 202, 204, 206, 208, 210 to a reset drain (not shown). FIG. 2(b) depicts charge storage regions 202, 204, 206, 208, 210 after the reset operation.
Unfortunately, some residual non-image charge 212 remains in charge storage region 206 after the reset operation. Manufacturing defects typically cause non-image charge 200 in charge storage region 206 to transfer incompletely to the reset drain. For example, misalignment of gates, defects in the semiconductor substrate, and added or missing materials can result in residual charge remaining in a charge storage region of a CCD after a reset operation. When residual non-image charge 212 remains in one or more charge storage regions of a CCD, an image sensor does not produce an accurate representation of a captured image.